In field husbandry, in principle the soil should be fertilized with nitrogen used in amounts corresponding to the quantity of nitrogen removed from the soil via harvesting. Fertilization recommendations are based on long-term average crop yields obtained from extensive test material. The recommendations ignore the fact that there are large variations between growing seasons in respect of the formation of crop yields. Water supply is often a minimum factor restricting growth, and when water is in short supply, the crops per hectare fall below the long-term averages. Furthermore, because Finland is situated at the northern limits of the cultivable area, from time to time there are fail-years, e.g. due to cold weather conditions, in which the crops remain considerably be-low long-term averages. In other countries as well, the crops per hectare vary correspondingly, due to variations in the growing conditions, in the first place weather conditions.
According to the fertilization recommendation for Finland, the average amount of nitrogen fertilizer to be spread on the cultivated area in mineral soil is 90-110 kg/ha. If the amount of fertilizer used is 110 kg/ha, the crop yield should be over 5000 kg/ha for the amount of nitrogen removed from the soil with the crop to correspond to the amount of nitrogen spread over the cultivated area in conjunction with sowing. Very often, however, e.g. when growth is limited by insufficient water supply, the crops per hectare fall as low as below 3000 kg/ha. In this case, a large overdose of nitrogen remains in the soil, and this nitrogen is liable to be washed away, denitrified, or it may combine with the soil in a form unusable for plants. The average crops in Finland in recent years have been slightly over 3000 kg/ha. Therefore, in the long term, the fields have been over-fertilized. In other countries, too, there are large variations in crops, depending on the weather and other conditions prevailing in each country and their changes. However, the problem resulting from over-fertilization is encountered in other countries as well, especially when there are unexpected changes in weather conditions and in the case of drought.
The fertilizer left unused because of over-fertilization, i.e. the production investment corresponding to the excess amount of fertilizer, has not yielded any returns In cultivation. In addition, the excess nitrogen is mainly washed away into water systems, resulting in a significant increase in the nitrogen load on water systems and therefore pollution. Moreover, in this case the fodder and food industries cannot get raw material of a quality consistent with the aim.